1. Field of the Invention
The present invention relates to a cab for a construction machine.
2. Description of the Related Art
Small-sized hydraulic shovels conventionally frequently used in civil engineering works, or truck cranes often used for assembling steel structures of small buildings in urban areas, must be operated in narrow places. Therefore, a turntable is mounted at the upper portion of the traveling gear. A working machine such as a shovel machine or crane machine is disposed on the turntable. An operator's cab (hereinafter simply referred to as the cab) is disposed at a position offset left or right from the working machine. The operator can get an unobstructed view of the front-end portion of the working machine. Furthermore, the cab is so designed not to protrude outside the radius of rotation of the outer fringes of the turntable. In this way, considerations are given not to interfere with surrounding structures and so on
Its first example is Patent Reference 1 (JP-A-2002-88812), for example. The following construction is shown in its page 3 and FIGS. 1, 3, 9, and 10: A cab is mounted offset left from a working machine. The cab is a slim and substantially rectangular parallelepiped except that the front window is tilted. Pillars are mounted at the four corners. An intermediate pillar is mounted in the outer side surface (in this case, left side surface), and an outwardly opening door is mounted to the intermediate pillar. Thus, a simple structure is constituted. Furthermore, construction details for realizing a decrease in the manufacturing cost are described.
In the above-described structure, however, in a case where the hydraulic shovel or truck crane is small-sized, there is the problem that it is difficult to mount a rectangular parallelepiped cab of necessary size within the radius of rotation of the turntable. Means for solving this is Patent Reference 2 (JP-A-2001-49696), for example. The following construction is described in its pages 5-6 and FIGS. 1-3: The outer side of the cab has a curved surface bulging outward along with the outer end of the rotating radius of the turntable. A sliding door is fitted to the outer side portion. This permits the cab to be positioned within the radius of rotation of the turnrable whether the door is opened or closed, and can secure necessary habitation space within the cab.
In FIGS. 8-10, a hydraulic shovel 70 has a traveling gear 71 the top portion on which is mounted a turntable 72 rotatably. A working machine 75 is mounted on the turntable 72. A cab 73 is disposed at a position offset left from the working machine 75. An engine compartment 74 is disposed immediately behind the cab 73.
The cab 73 has an inner side surface portion 78 and a rear surface portion 79 comprising vertical planes. A front surface portion 76 has a lower portion 76a comprising a vertical plane and an upper portion 76b comprising a rearwardly tilting plane. An outer side surface portion 80 is formed by a curved surface bulging outward substantially along the outer end of the rotating radius of the turntable. The outer side surface portion 80 has a front lower portion 80b that is a cylindrical, two-dimensionally curved surface and a front upper portion 80a that is a three-dimensionally curved surface having as its base end the cylindrical surface of the front lower portion 80b. The radius of curvature of this three-dimensionally curved surface decreases from the base end upwardly in step with the rearward tilting of the front upper portion 80a. A rear portion 91 is cylindrical, a two-dimensionally curved surface having a radius of curvature smaller than that of the upper-end portion of the front upper portion 80a. 
The outer side surface portion 80 has a front-end portion, a rear-end portion, and an intermediate portion in which a front pillar 82, a rear pillar 83, and an intermediate pillar 81 are respectively disposed. A sliding door 86 is disposed in an entrance opening 84 between the front pillar 82 and intermediate pillar 81. The surface of the sliding door 86 is shaped to conform with the curvature of the front lower portion 80b and front upper portion 80a of the outer side surface portion 80 of the cab 73. Rollers (not shown) mounted inside the top and bottom of the front-end portion of the sliding door 86 and rollers (not shown) mounted toward the center and inside the top and bottom of the rear-end portion of the sliding door 86 are engaged over guide rails 89, 90, and 85, respectively. This permits the sliding door 86 to be slidable along the outer side surface portion 80 of the cab 73.
In the construction shown in FIGS. 8-10, the radius of curvature of the cylindrical, two-dimensionally curved surface of the rear portion 91 of the outer side surface portion 80 of the cab 73 is made smaller than that of the upper-end portion of the front upper portion 80a. Consequently, the sliding door 86 can slide while in almost complete contact with the outer side surface portion 80 of the cab 73. Accordingly, when the sliding door 86 is opened, the door 86 does not protrude outside the radius of curvature of the outer fringes of the turntable 72. Furthermore, a large indoor space can be secured in the cab 73.
However, the prior art cab disclosed in Patent Reference 2 has the following problems.
(1) The cab 73 has the four pillars at the four corners (i.e., front, rear, left, and right corners) and has one pillar at the bulging portion of the outer side surface portion 80 (only outer pillars 81, 82, and 83 are shown). Therefore, there are many pillars. The structure is complex. Consequently, the manufacturing cost is high.
(2) In spite of having the five pillars, only one intermediate pillar 81 is present in the center of the cab 73. Where the cab 73 of this structure is so constructed that when the hydraulic shovel 70 rolls over, the cab is strong enough to withstand the crush and secure a required space inside (hereinafter referred to as the rollover protection strength) it is necessary to reinforce all of the pillars at the four corners of the cab 73 and beams connecting top portions of the pillars (only outer beam 87 is shown). Hence, the manufacturing cost of the cab is quite high.
(3) Another conceivable method to give the cab a structure having rollover protection strength comprises mounting a separate gate-shaped rollover guard across the cab. In this case, however, the cost of the rollover guard is produced. In addition, there arises the problem that the rollover guard protrudes outside the radius of rotation of the turntable 72. That is, it is required that the cab and rollover guard be contained within the radius of rotation of the turntable.